1. Field of the Invention
The present invention relates generally to an ultrasonic inspection method of a square billet and more particularly to an inspection method for effectively detecting internal defects including subsurface defects of the square billet throughout the inside and the entire surface layer (to a depth or several tens mm) in an on-line system and with high speed.
2. Description of the Prior Art
In the secondary conditioning industry of wire rod and steel bar processes have been implemented recently within the intention of saving labor and energy and reducing costs. As a result, the forming conditions of wire rod and steel bar became more difficult. Therefore cracking of work during cold forgoing and breakage during wire expanding caused by the inclusion of small foreign substances have become problems. In order to solve such problems, smelting techniques have been established which remove unwanted material outside of the furnace and prevent it from entering during pig iron manufacture and steel manufacture. This technique together with inspection techniques to detect the existence of small inclusions of foreign material has become essential from the viewpoint of quality assurance.
In regard to inspection techniques, ultrasonic inspection methods of a steel bar product are known where the steel bar is rotated or a probe is rotated around the steel bar and a focused ultrasonic beam enters the specimen. Similar inspection methods cannot be applied to a wire rod product on account of the small diameter, and inspection of the coiled final product throughout its whole circumference and length is impossible at the present time.
Inclusion of foreign matter in wire rod and steel bar, which causes problems, is present already in the raw material stage. Therefore, if inclusion is detected in the step of forming a square billet, the inside quality of the product can be assured. Particularly in the case of wire rod, ultrasonic inspection of the inside of a square billet may be utilized as a substitute for internal inspection of the product. Introduction of inside inspection techniques at an intermediate stop in the process permits a defective billet to be removed earlier. On account of the small length of the specimen and good inspection efficiency compared to inspection of the final product, inspection at an intermediate step has excellent efficiency. On the other hand high detecting accuracy is required even to detect small inclusions in order to assure reliability.
One ultrasonic inspection method of a square billet known in the part art, is the normal beam technique using a double crystal probe. In this method, a defect inside of a square billet can be detected but a subsurface defect cannot be detected. A subsurface defect is one just below the surface of the specimen. While technically a part of the interior rather than the surface, the fact that it lies just below the surface makes it difficult to detect by some methods. It is sometimes considered a separate type of defect from other internal defects because of this detection problem. When a square billet is machined into a product, an inclusion which causes the work to crack or the like exists frequently at the subsurface layer. Therefore the establishment of inspection methods for detecting internal defects including subsurface defects is required. Ultrasonic inspection methods of a square billet using mechanical scanning are also known. These methods are not suitable for high-speed manufacture which needs high-speed movement of the probe.